A backplane generally comprises a printed circuit board having a number of card connection slots or bays. Each slot or bay comprises, e.g., one or more modular signal connectors or card edge connectors, mounted on the backplane. A removable circuit board or “card” can be plugged into the connector(s) of each slot. Each removable circuit board contains drivers and receivers necessary to communicate signals across the backplane with corresponding drivers and receivers on other removable circuit boards.
One or more layers of conductive traces are formed on and/or in the backplane. The traces connect to individual signal connection points at the various slots to form data lines and control lines.
Router backplanes present a challenging area of circuit board design (for convenience, routers and switches will be referred to herein collectively as “routers”, as the technical distinctions between the two are unimportant to the invention as described herein). By their very nature, configurable modular routers require a high degree of interconnectivity between their removable router cards. With any appreciable number of cards, it becomes infeasible to build large parallel point-to-point connection buses between each pairing of the cards. This limitation hinders further growth in large router throughput, as the next generation of large routers may well see throughput requirements measured in terabits-per-second. As such throughput requirements may require several tens (or even hundreds) of logical ports to exchange data simultaneously at twenty to one-hundred Gigabit-per-second (Gbps) speeds, it can be appreciated that the connectivity and throughput requirements placed on large router backplanes are extreme.
Many router manufacturers, believing that the limits of electrical circuit boards have been reached in the area of large router backplanes, are now designing optical backplanes for their next-generation products. Optical backplanes avoid some of the most problematic characteristics of electrical backplanes, such as trace density, signal attenuation, signal reflection, radiated noise, crosstalk, and manufacturing limitations-characteristics that become increasingly significant as single-trace signaling speeds push into the multi-Gbps range. Optical backplanes, however, come with their own set of problems, chief among these being cost and complexity.